I. Field of the Invention
This invention relates to starch based alkaline curing adhesive compositions specifically intended for use in the manufacture of corrugated paperboard.
II. Brief Description of the Prior Art
In the production of corrugated board the procedure usually employed involves a continuous process utilizing a strip of paperboard corrugated by means of heated fluted rolls. An adhesive is applied to the tips of the protruding flutes on one side of the corrugated strip, a flat strip of another paperboard is then brough into contact with these tips, and, by the application of heat and pressure, a bond is formed. This procedure is then repeated on the other side of the corrugated strip so as to yield a hard sheet of paperboard comprising an inner corrugated layer contained between two flat outer layers.
The adhesive composition most widely used in the above process comprises raw, ungelatinized starch suspended in an aqueous dispersion of cooked starch. The cooked starch dispersion, being rather viscous, serves as a carrier to keep the ungelatinized starch in suspension. At the point in the corrugating process where the flutes of the corrugated paperboard are to be adhered to the flat paperboard, sufficient heat is applied to gelatinize the raw, ungelatinized starch component of the adhesive, resulting in a sudden rise in viscosity and the formation of the adhesive bond.
Since it is often desired or necessary that the corrugated adhesive yield waterproof or water resistant bonds, several procedures have been devised to produce potentially waterproof adhesive composition for use in the manufacture of corrugated paperboard. In one such procedure, amylaceous material has been combined with urea-formaldehyde resins using aluminum sulfate as a catalyst to produce waterproof or water resistant bonds. These compositions must be prepared at a pH of less than 5 in order to enable the resin to function as the waterproofing agent. This low pH, however, imparts poor flow properties to the adhesive composition, thus resulting in greatly reduced machine speeds, corrosion of the glue rolls, and prevention of the development of many desirable paste characteristics such as tackiness, viscosity, rate of setting and other properties. In another method, amylaceous material is cooked, under alkaline conditions, with urea-formaldehyde resins which set or harden at a pH of above 7. Although the bonds of corrugated board prepared from these adhesives show some improvement in water resistance, on exposure to wet weather conditions they nevertheless tend to delaminate and fail to pass U. S. Government specifications for water resistant corrugated paperboard.
Other waterproofing corrugating adhesives, such as those employing resorcinol and formaldehyde with starch, similarly suffer from serious drawbacks. The cost of these adhesives is often prohibitive because of the large amount of resorcinol that is required. In addition, these adhesives have a short pot life at the pH range most useful for maximum production speeds. Other shortcomings of this adhesive include the slow rate of bond formation which results from its incompatibility with borax which is often added to starch-based adhesives as a tackifying agent to improve machine speeds.
Still other waterproofing corrugating adhesives employ the reaction product of acetone with formaldehyde as the waterproofing agent. A particular weakness of these adhesives is their short pot life. Thus, when acetone-formaldehyde condensates are added to conventional starch-based adhesives and the mixtures allowed to age, the wet strength of bonds obtained after 24 hours aging of the adhesive is considerably lower than after 4 hours aging. The practical consequence is that the corrugated paperboard manufacturer is frequently forced to discard adhesive that has not been used within a certain period of time.
There is thus a need in the art for a crosslinking agent for use in corrugating adhesive compositions which will exhibit wet strength performance properties superior to those found in commercially available materials, which will maintain such superior properties even after 24 hours aging, and which will be easily and economically prepared.
It is therefore an object of the present invention to provide economical corrugating adhesive compositions which possess superior wet strength performance and which remain stable for extended periods of time.